1. Field of the Invention
The invention relates to traction drives, that is, to drives or power transmissions in which the power is transmitted by means of drive members which are in frictional driving contact with each other.
2. The Prior Art
Traction drives have been known for some time and have been used in various applications. In principle, such drives are attractive from the engineering point of view because the power is transmitted by means of smooth metal rolling elements which can be moved freely with respect to one another to provide infinitely variable transmission ratios. Traction drives are also inherently quiet, with low vibration levels and produce no torsional pulsations.
On the other hand, traction drives with infinitely-variable speed ratios have generally been restricted to low power applications because they tend to slip under even moderate loads. The small contact areas necessitated by low spin requirements in infinitely variable traction drives lead to high contact pressures, especially in heavier duty applications, and this induces rolling contact fatigue which results in premature failure of the rolling elements.
Recent studies have shown that wide-range variable ratio traction drives could be attractive for automotive use since they could be capable of reducing fuel consumption by keeping the engine operating at its maximum efficiency. In addition, they are particularly attractive for use with prime movers such as turbines which are sensitive to torsional vibration.
A review of existing traction drive technology is given in the article "New and Better Traction Drives are here", R. W. Carson, Machine Design 46 (10), 148-155 (1974).
The infinitely-variable ratio traction drives in general use at the present employ point or elliptical contact to transmit traction forces. This means that contact stresses will be high or, if the size of an elliptical contact is increased to reduce the stress, the spin will also be increased with a consequent reduction in the life of the rolling elements. For example, the toroidal drive described in the article by R. W. Carson has shown the most promise in power handling capability. The size of the elliptical contact formed at the toroidal roller contacts is limited by the spin in the contact for a given range of speed ratios. For a useful ratio range the contact ellipse has a limiting size and the output torque can only be increased by increasing the contact stress. Excessive contact stress levels cause severe life reductions of the rolling elements due to contact fatigue. A balance must therefore be found between wear caused by spin and fatigue caused by high contact stress. This balance limits the power output of the toroidal type of traction drive. The toroidal drive may achieve line contact between the rolling elements but only with a very limited range of speed ratios.